Contemporary evolutionary theory was largely inadequate to explain many types of altruistic behavior, in which individuals sacrifice themselves or their fitness toward group benefit, until modified by Hamilton in 1964. The theory that he proposed is based on the fundamental concept that the fitness of genotype depends not only on an individual's own properties but also on its interactions with other individuals with the same and other genotypes. This expanded concept of fitness was termed "inclusive fitness". The process of selection on the basis of inclusive fitness was termed "kin selection". Although many data are on hand that support the kin-selection theory, they are incomplete because they were not gathered systematically to test the applicability of the concept to specific cases. Certain species of jays are well suited for testing the inclusive fitness concept because of their social organization and the ease with which individuals can be captured, marked, and recognized in the field. The most accessible of these species, the Mexican jay, displays various forms of altruistic behavior, including group defense of territory, communal harassment of predators, and a moderate amount of communal participation in nesting and care of young. It is proposed to obtain quantitative estimates of the following crucial kinds of evidence: 1) the genetic relationships among altruists and other group members, 2) the frequency and duration of altruistic acts in feeding young and in flock defense, 3) the risks incurred by these altruistic acts, 4) the benefits to the group from the altruistic individuals, and 5) group fitness, in terms of relative success of different groups in recruitment to subsequent generations. With these items of information at hand the kin selection theory can be more accurately assessed than has been possible before in a diploid species.